According to “Ministerial Ordinance Regarding Compositional Standards, etc. of Milk and Dairy Products”, fermented milk is defined as a product obtained by subjecting milk or a milk-like substance or the like containing solid non-fat in an amount not less than that of milk to fermentation with a lactic acid bacterium or yeast to form a paste or a liquid, or a frozen product thereof. The fermented milk is broadly classified into: (a) hard yogurt (i.e. solid fermented milk or set type yogurt) obtained mainly by filling its material into a container and then fermentating and solidifying the material in the container; (b) soft stirred type yogurt (i.e. pasty fermented milk) obtained by fermentation in a large tank or the like, and subsequent curd pulverization and optional mixing with pulp, sauce, or the like, followed by filling the obtained mixture into a container; and (c) drinkable yogurt (i.e. liquid fermented milk) obtained by fine crushing of hard yogurt or soft stirred type yogurt with a homogenizer or the like to enhance properties as a liquid, optional mixing with fruit pulp, sauce, or the like, and subsequent filling into a container.
According to the compositional standards for fermented milk in Japan (Ministerial Ordinance Regarding Compositional Standards, etc. of Milk and Dairy Products), the ratio of solid non-fat (i.e. milk solid non-fat) should be 8.0% or more, and the number of lactic acid bacteria or yeast cells per mL should be 10,000,000 or more. Further, according to the international standards for yogurt defined by FAO/WHO, yogurt is defined as follows: “Yogurt is a coagulated dairy product obtained by subjecting milk or a dairy product to lactic acid fermentation through actions of Lactobacillus bulgaricus and Streptococcus thermophilus. Additives (e.g., milk powder, skim milk powder, whey powder, and the like) can be used optionally. Microorganisms as described above in the final product must be viable and abundant.”
Because fermented milk contains viable cells such as lactic acid bacteria, an increase in acidity and a decrease in pH occur owing to lactic acid and the like to be generated by the lactic acid bacteria when the fermented milk is stored for a long period of time. Therefore, owing to the increase in acidity and the decrease in pH over time, it has been difficult to keep flavor and quality constant as compared to those of a product immediately after production.
In order to solve this problem, various methods have been proposed heretofore. For example, there is a proposal concerning a method for producing yogurt containing viable lactic acid bacterium cells which comprises adding a lactic acid bacterium to a yogurt material composition to control a fermentation degree of milk in the composition to a desired one; leaving the resultant to stand at low temperature, followed by heating under such temperature and time conditions that the temperature is not lower than the limit temperature for growth arrest of the lactic acid bacterium at the high-temperature side but complete killing is not achieved; and cooling the resultant (see Patent Literature 1). Further, there is a proposal concerning fermented milk containing chitosan by which an increase in acidity can be suppressed (see Patent Literature 2).
The technology described in Patent Literature 1 involves the heating treatment after the fermentation step, and has problems in that setting and adjustment of the heating conditions are complicated, excessive supply of heat energy is required, and the flavor of yogurt may be deteriorated owing to the heating treatment. In addition, the technology described in Patent Literature 2 involves using chitosan, which is not contained in general fermented milk, as an additive, and hence has a problem in that the flavor of the fermented milk may be changed.
In addition, as other means for suppressing production of acids, there have been known, for example, a method involving adding a peroxidase (see Patent Literature 3), a method involving using Lactobacillus helveticus and Lactobacillus acidophilus in combination (see Patent Literature 4), a method involving giving a heat shock at a predetermined temperature for a predetermined time after the end of fermentation (see Patent Literature 5), a method involving using a low-temperature-sensitive lactic acid bacterium (Patent Literature 6), a method involving adding Lactococcus lactis subsp. Lactis capable of producing nisin (Patent Literature 7), and a method involving maturing a fermented product at an ice temperature range after the end of fermentation (Patent Literature 8).
However, each of those methods has problem in that another additive such as the peroxidase is further added, applicable lactic acid bacteria are limited, or production may be complicated by introducing an additional step such as the heat shock treatment or the ice-temperature maturation. Yogurt is supported by consumers because of its natural and refreshing flavor obtained from its moderate acidity. However, when the acidity is suppressed during storage of the yogurt by the conventional technologies, it cannot be avoided that its natural and refreshing flavor is lost.
On the other hand, there is known a method involving liquefying gel-like fermented milk obtained by fermentation of milk by homogenization at a homogenization pressure of 100 to 150 kg/cm2 (i.e. 10 to 15 MPa) (see Non-Patent Literature 1). However, the purpose of the method is to crush the cooled fermented milk finely to liquefy the milk.